In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in “n type” extrinsic material wafers, whereas if “p type” extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
Dosimetry is the measurement of ions implanted in a wafer or workpiece. In controlling the dosage of implanted ions, closed loop feedback control systems typically are utilized in order to dynamically adjust the implantation to achieve uniformity in the implanted workpiece. Such control systems utilize real-time current monitoring to control the workpiece slow scan velocity. A Faraday disk or Faraday cup periodically measures the beam current and adjusts the slow scan speed to ensure constant dosing. Frequent measurement allows the dose control system to respond quickly to changes in beam current. The Faraday cup is located close to the workpieces, making it sensitive to the beam current actually dosing the workpieces.
The purpose of the dosimetry system is to know the amount of dopant being delivered to a workpiece and, in ion implantation applications, it is done by measuring electrical current (i.e., the beam current). If all the dopant particles carry the same charge value q, the number of dopant particles “n” delivered to a wafer per second is given simply by the measured electrical current (beam current) “I” (amps) as follows:n=i/(qe),where e is the value of electron charge, which is about 1.6×10−19 Coulomb. Normally all the ions carry a same charge value and the charge value, q, is a single integer. If the ion beam consists of ions of different charge states (including neutrals for which the charge value is zero), q is a weighted average of the charge values with its charge state distribution and the simple relationship provided above no longer holds. Since the charge state distribution of an ion beam can change (by charge exchange reactions which will be described in greater detail infra) and is very difficult to measure, especially since it may contain large portion of neutral atoms (which can not measured with any electrical methods), substantial effort is paid to keep the charge value in the ion beam at the initially intended single value.
Some processes, however, act to cause ions to change their initial charge value, and one such process is called a charge exchange reaction. When a high speed ion comes in close proximity to another molecule or atom, the ion may pick up an electron from the molecule or atom (i.e., an electron “picking up” reaction), or may loose an electron to the molecule or atom (i.e., a charge stripping reaction). The former reaction reduces the value of ion charge by 1, for example, a singly charged ion becomes a neutral, that is, an electrically neutral atom. The latter increases the value of ion charge by one, e.g., a singly charged ion becomes a doubly charged ion.
In ion implantation systems, effort is expended to prevent the frequent occurrence of these charge exchange reactions by maintaining the entire path of the ions at a high degree of vacuum, typically at <1×10−6 torr. However, in many ion implantation applications dealing with semiconductor manufacturing, a workpiece, a semiconductor wafer, is partially covered with a thin organic film called photoresist to mask certain areas and thereby selectively dope only a desired portion of the wafer. When high speed ions hit the photoresist layer on the wafer, some of molecular bonds in the organic film are broken and some of the released atoms form gas, most likely hydrogen gas. The amount of released gas can be substantial and may operation to degrade the vacuum level in the ion beam path, and in extreme cases almost 50% of the ions in the ion beam experience charge exchange reactions.
For each charge exchange reaction, there is a value called reaction cross section which describes a probability of the occurrence of the reaction under a unit density of residual atoms. The reaction cross section is given in the dimension of area (as its name suggests, usually in cm2), and its value changes in wide range by ion speed, ion charge value, ion mass and a residual gas atom. If one denotes the value of charge exchange cross section, σxy, for a reaction to change ion charge x to y, the fraction of the ion beam that has changed charge value from the original charge x to a final charge value y after passage through a gas layer is given as;fy≈3.3×1016p*L*σxy,where p is a vacuum pressure in torr, and L is the length of the passage in cm. The fraction of the original charge state x is given as:fx=1−(fx−1+fx−2+ . . . )−(fx+1+fx+2+fx+3+ . . . )and the second term on the right is for electron “pick up” reactions, while the third term is for stripping reactions.
Using the fraction of final charge, fy, one can calculate the average charge value after the passage of the gas layer as:qav=fx*x+{fx−1*(x−1)+fx−2*(x−2)+ . . . }+{fx+1*(x+1)+fx+2*(x+2)+ . . . }.For practical purposes, one can limit the final charge states to values between 0 and 3. For example, for a starting ion charge of +1 (x=1),qav≈(1−(f0+f2+f3))+{f0*0+f2*2+f3*3}.Further, when the ion beam energy is low enough for all the stripping reactions to be negligibly small (σ12≈σ13≈0) and the only charge exchange reaction is an electron picking-up, the formula becomes much simpler:qav≈1−f0.In this simplified example case, the formula for number “n” of dopant atoms is given by the measured beam current i:n=i/((1−f0)*e),i.e., number of dopant atom goes up by 1/(1−f0) for a same beam current.
The above example shows that to get actual number of dopant from a measured electrical beam current, we have to know f0, the fraction of charge exchanged ions, which is very difficult to know.
FIG. 1 shows a prior art ion implantation system, that employs pressure compensation for dosimetry control. An ion beam 9 exits an ion source 2, and is mass analyzed by a mass analyzer 3, and then directed toward an end station 5 that, in one example, is a batch system containing a plurality of workpieces 6 thereon. A Faraday cup 7 measures the ion beam reaching the wafers immediately behind the end station through a slit 8 on the disc. Since number of dopant particles arriving at the disc has to be calculated from a measured beam current at the Faraday 7 with the above formula that contains a factor f0 which in turn depends on the pressure in the beam path, this method uses an empirical correction to the measured beam current with an instantaneous pressure measured with an ion gauge 16 placed in a process chamber 15.
In this method, a proportionality factor between a beam current and number of atoms on wafer is empirically determined for each implantation condition, according to each implantation “recipe”, that is: ion beam energy, mass, charge value, beam current and total dose level of the implantation. One shortcoming of this p-comp method is that the empirical factor has to be determined on each implant recipe prior to actual implantation and effort has to be made so that the factor stays unchanged for long time. Another challenge to this prior art solution is that the assumed functional approximation between a pressure and dose tends to break down at higher pressures. Because of this challenge, some users limit beam current to keep the approximation valid although it affects the productivity adversely.
Accordingly, it is desirable to have an improved system and method for performing dosimetry control.